The application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9802169-4 filed in Sweden on Jun. 17, 1998; the entire content of which is hereby incorporated by reference:
The invention relates to a communications network and also to an add and drop node and an error handling method in such a network.
There exist different types of communication networks, with different ways of taking care of errors. There also exist different types of multiplexing in communication networks, such as frequency division multiplexing (FDM), time division multiplexing (TDM) and wavelength division multiplexing (WDM).
Optical multi-channel systems employing wavelength multiplexing are used both in new networks and in order to enhance the transmission capability of existing optical fiber networks. Thus, information channels that previously had to be transmitted on a plurality of separate fiber pairs can be forwarded in a single fiber pair in WDM networks.
Using optical wavelength division multiplexed channels means that a plurality of serial information signals, i.e. a plurality of serial binary signals, are transmitted on the same optical fiber. Each serial signal is modulated on a carrier of its own, having a definite wavelength clearly separated from the other carriers. These modulated light signals are then combined in an optical coupler or optical multiplexer arrangement to a composite light signal on the considered optical fiber. The signal modulated on a carrier together with the carrier can be called a channel or traffic channel.
Optical wave length multiplexing can be used in different optical fiber network configurations or architectures. Such an architecture is the FlexBus(trademark) concept as described in B. S. Johansson et al., xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d, Electronics Letters, Dec. 5, 1996, Vol. 32, No. 25, and U.S. Pat. No. 5,680,235, this architecture comprising a ring configuration of optical links connecting a plurality of nodes. The FlexBus(trademark) concept has emanated from the need for protecting ring networks against fiber cuts and optical amplifier failures, and to solve the problem, often associated with optical ring network architectures, of circulating signals and noise.
In the FlexBus(trademark) architecture one section or segment of the fiber ring is always made passive or inactive by means of optical switches or amplifiers. This intentionally introduced break effectively eliminates all problems associated with circulating signals and hence allows that less circuit components having lower performance can be used. In the case of a real failure of a section or link, the segment being intentionally made inactive is logically xe2x80x9cmovedxe2x80x9d from its former position to the failed section, by making the inactive segment active and the faulty segment inactive, what also is called that xe2x80x9cthe bus flexesxe2x80x9d, and thereby the traffic is restored.
Different developments of the FlexBus(trademark) may be found in for example WO 96/31025 and WO 96/24998.
One problem with FlexBus(trademark) and many other error handling algorithms in communication networks is that they can only take care of one error at the time. In FlexBus(trademark) this is due to the fact that when a deactivated segment contains a fault it cannot be logically moved as described above until the fault is repaired, because else the bus would not work.
An object with the invention is to solve this problem by using protection links in parallel with the ordinary links. If a fault occurs the deactivated segment will logically move to the fault as usual. A bypass connection is made over protecting links parallel to the first deactivated segment and a second deactivated segment is created instead of the now deconnected deactivated faulty segment.
Since the second deactivated segment does not contain any fault it is possible to logically move in the case of the occurrence of a second fault. When the second deactivated segment has logically moved to the second fault a third deactivated segment will, in the same way, be created in parallel to the second deactivated segment. Thus, the third deactivated segment is possible to logically move in the case of a third fault etc. This is of course not extendable forever. It depends on the look of the net of protection links and it could eventually happen that the new fault occurs in a place where it is not remediable. It rarely occurs more than one or two errors at the same time though, so in practise there will be no problem.
The advantages with this are that a simple and cheap error handling method is provided, which is capable of taking care of many occurring errors.
If the links are also physically parallel a break in a protection link will probably occur at the same time as a break in an ordinary link. This problem is solved in an embodiment of the invention in that multi direction cross points are used to connect different protecting links in a more flexible way, forming a sort of net.
The advantages with this embodiment are that it is safer, that already existing links may be used as protecting links and that a more flexible bus reconfiguration is possible in the case when a larger part of the ordinary ring is faulty.